Electrical Systems of Renewable Energies

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Electrical Systems of Renewable Energies

BMEVIVEM262

Károly Veszprémi Mátyás Hunyár

István Vajda

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Electrical Systems of Renewable Energies

by Károly Veszprémi, Mátyás Hunyár, and István Vajda Publication date 2012

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Chapter 1. chapter 1.

1. 1. dia

2. 2. dia

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chapter 1.

3. 3. dia

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chapter 1.

Wind power plants convert the kinetic energy of wind into electric energy. Two types of these have been developed according to the relative position of the axis of the rotation to the direction of the wind (or to the surface of the Earth):

1. horizontal axis wind turbines (HAWTs) have their axes parallel with the wind direction (see Fig. 2-1), 2. vertical axis wind turbines (VAWTs) have their axes perpendicular to the wind direction.

The concept of wind power plants, windmills and wind farms.

Wind turbines convert the kinetic energy of the wind into rotating work. Different types of HAWTs can be seen in Fig. 2-2, some types of VAWTs can be seen in Fig. 2-3.

4. 4. dia

In the case of HAWTs, the area swept by the blades has to reposition perpendicular to the variable wind direction to extract maximum energy. In the high power range the rotation of wind turbine (and the nacelle) is implemented by servo drives. A further drawback of this type is that the heavy gear box and the electrical generator have to be placed at the top of the tower (in the nacelle) which requires a stronger construction and higher expenses. In spite of this almost exclusively HAWTs (doubly bladed or three bladed) are used nowadays

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chapter 1.

The drawbacks of VAWTs in comparison with the HAWTs are:

1. aerodynamic efficiency is lower (at the same useful power the dimensions and the expenditures are higher), 2. the output power cannot be controlled, or can only be controlled with difficulties,

3. in the case of Φ Darrieus turbine the manufacturing, transportation and set up of the blades are fairly difficult.

Some advantages of VAWTs in comparison with the HAWTs are:

1. it does not demand yaw mechanism in spite of variable wind direction,

2. the electrical generator can be placed near ground level which results in a cheaper tower and a simple set up, 3. the stress caused by the pull of gravity does not change during rotation.

In the case of low power level and/or extra severe circumstances the VAWTs may be competitive compared with the HAWTs.

Further we shall discuss only the double or three bladed HAWTs, with few exceptions.

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chapter 1.

In the engineering practice the vertical mean wind shear profile can be given with a power low characteristic.

This gives a good approximation up to the height required by the wind turbines:

(2-1)

Unfortunately α the so called Hellmann exponent is not constant, but it depends on the site (the terrain, the surface roughness, the shading), and the temperature stability (therefore it has a changing daily and yearly course).

In the case of information measurements the cup anemometers are mounted at two different heights on the 35- 40m high anemometry mast. In the case of measurements for deployment the measurements are carried out at three different heights e.g. at 40-60-80m and two different Hellmann exponents are calculated for the two height domains. With the more expensive SODAR (Sound Detection And Ranging) device, which emits and receives sound and from that infers the wind speed at different heights using the Doppler shift principle, the observable range is between: 30-315m and its vertical resolution is 15m.

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chapter 1.

The kinetic energy of any “m” particular mass of moving air is:

(2-2)

Consider “m” the mass which passes through the “A” area with perpendicular “v” velocity during “t” time:

(2-3)

Here ”ρ” is the density of air, which is 1.223kg/m³ standard circumstances.

The power contained in the wind (if wind velocity is constant during the short “t” time) is:

(2-4)

As wind velocity increases by positioning the hub of the wind turbine higher wind power plants are being built with continually growing tower heights (see Fig. 2-5).

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chapter 1.

The power given by (2-4) cannot be extracted from the wind because of the aerodynamic efficiency (which is lower than 100%) and other losses. Before the detailed calculations it is practical to investigate qualitatively the changes in the characteristics of air passing through the turbine. In the interest of easier overview it is practical to imagine the wind in a widening stream tube. In this case the exchange of energy between the inner and outer air masses can be ignored (see Fig. 2-6). It is a good approximation that air cannot be compressed by small specific energy change (work) that is its density is constant. Then the mass of moving air per second (the mass flow rate) are identical at any cross-section of the tube (continuity):

(2-5)

The air arriving to the turbine is slowed down and is congested, so in accordance with (2-5):

Before the turbine there is no work performed (the sum of energies is unchanged):

The static pressure of air can suddenly decrease passing through the turbine-swept surface, but a step change in

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chapter 1.

Let us imagine an actuator disc with surface instead of the turbine in Fig.2-6. The change of momentum of air causes a force, which comes entirely from the pressure difference across the actuator disc. The Bernoulli’s equation is applied separately to the upstream and downstream section of the stream tube.

One should recognise the general rule, that the half of the axial speed loss in the stream tube takes place upstream of the actuator disk and half downstream. As the force F is concentrated at the actuator disk, the rate of work (the power) done by the force is proportional with the speed of the virtual movement of actuator disk.

The Betz limit (Cp =0.593) cannot be reached with actual wind turbines because of the additional aerodynamic losses. These losses can be taken into account only with a more precise theory which consider the tangential wind speed component (e.g. by Schmitz theory).

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chapter 1.

The relative speed loss introduced in the previous, 2-3-2 paragraph:

(2-7)

is useful for theoretical considerations and for simple demonstration. However it is not suitable for actual optimisation, because the velocity is not measurable or can be measured with difficulties (see paragraph 2-3-4).

Therefore a similar characteristic parameter had to be found, which can be calculated from easily measurable quantities and at the same time it contains the rotation of the wind turbines. This parameter is the tip-speed ratio:

(2-8)

By the accomplished measurements it has been proven, that the has a maximum value in function of, too. The position of this maximum is dependent on the type of turbine (see Fig. 2-7).

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chapter 1.

In the case of HAWT’s it is evident the characteristics observed that the optimal tip speed ratio is inversely proportional to the number of blades. We can distinguish high speed (λopt≈5~10) and low speed (λopt≈1, max.

λopt≈3) wind turbines. In the high power range almost exclusively high speed turbines are used for generating electricity. This is because the high speed turbines require gear boxes with lower gearing ratios (with smaller sizes, lighter weights and lower costs). Namely the optimal rpm of the traditional electrical generators is given at around 1500/min.

The optimal CP and λ values apparently contradict the simple physical consideration, that the more air the blades are in contact with the more energy/power can be extracted from the wind. As in the case of wind turbines with few blades apparently a large amount of air can pass between the blades unobstructed, the multi-bladed turbines would have to have better power factor at any rpm (at any tip speed ratio).This contradiction can only be solved with a theory more precise than the momentum theory, one which considers the rotation of turbine and the number of blades, too (see Fig. 2-8).

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chapter 1.

An equal but opposite torque acts on the air as that of the torque of the turbine because of the theory of action- reaction. Therefore a wind speed component in tangential direction (v2t) arises, too. As a result of the two wind speed components (v2t, v2ax=v2 ) the air downstream the turbine leaves in a form of a helical path (designated with in Fig. 2-8). As it will be proven in the next 2-3-5 paragraph the pressures are different at the top and bottom surface of the blades, therefore other flows are established also around each blade which are designated with in Fig. 2-8.

If any blade of turbine gets into the vortex of the preceding blade, then the direction of the force acting on the blade will be different from the designed (optimal) direction, and the torque acting on the turbine will be lower than the attainable value (it can even drop down to zero). In optimal case the whirling air has to leave in axial direction just before the arrival of the subsequent blade. Namely the blade rotating with w angular velocity has to perform

the rotation angle of ε=360°/z in a little longer time than the time during which the vortex

can leave the area swept by the turbine with speed. With this condition it can be proven that in the case of very narrow (slender) blades:

(2-9)

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chapter 1.

Hereafter we will examine the elementary forces acting on a blade element with dr width (see Fig.2-9). The flow around it, the wind velocities and characteristic angles are outlined in Fig.2-10.

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chapter 1.

α: angle of attack: between the chord line and relative wind speed, β: pitch angle: between the chord line and rotor axis,

ϑ: pitch angle: between the chord line and plan of rotation, δ: angle between the relative wind speed and plan of rotation,

(2-11)

The air masses above the blade element have to flow much more swiftly than the air masses below it to arrive the trailing end of the airfoil at the same time. In accordance of Bernoulli’s law an increase of pressure below the blade and a decrease of pressure above it are developed. A dFE lift force acting on the blade element will be the result of the two effects, which is perpendicular to the resulting relative wind speed. (It has particularly great importance for aeroplanes.) The larger part of the lift force is given by the suction.

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chapter 1.

In the case of an aeroplane one has to strive for high dFE/dFV ratio (see Fig.2-11) in the interest of high loadability and an engine with lower required power. According to measurements in wind tunnels the elementary forces acting on the airfoil element are proportional with the square of relative wind speed, air density and the area (tBdr) facing the wind direction (see Fig.2-11). Adequately large dFE/dFV ratio can be provided with high CE(α)/CV(α) lift-drag coefficient ratio. In the case of aeroplanes this can be achieved on the one hand with carefully selected low α angle of attack, on the other hand with asymmetrical (and cambered) airfoil. The question arises: are these statements valid for wind turbines too?

With wind power plants it is not the dFE lifting and dFV drag forces that are interesting but the dFt tangential and dFax axial elementary forces as these latter two determine the torque and the stress on the structure respectively.

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chapter 1.

The history of aviation goes back many decades and enormous experience has been accumulated in this field.

The considerable similarity provides an opportunity to translate the results from one field of research into the other. In the course of measurements carried out in wind tunnels (and in aviation) from the angles indicated in Fig. 2-10. (in the case of wind turbine) only α angle of attack has meaning. All the forces dFE, dFV and dFt, dFax

effect in the same plane, the plane of blade element. (This plane is horizontal in the case of Fig. 2-9.).The reference frame in the case of dFE, dFV is defined by vr relative wind speed, while in the case of dFt, dFax it is determined by the plan and axis of rotation, according to Fig. 2-10. The transformation among the components of the elementary forces is based on the fact, that the resultant force acting on the blade element is the same irrespectively of the used coordinate system. The tangential force in Fig. 2-12. determined by (2-13) is given by this transformation in view of dFE and dFV.

The torque on the complete rotor can be calculated by numerical integral according to (2-14). The Ro radius in the formula can be understood by Fig. 2-9.

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chapter 1.

With knowledge of the torque the power coefficient can be calculated using up PO and w (see (2-15)). The CP

values calculated this way are the functions of α angle of attack and δ angle. It is difficult to measure these angles and they can only be changed indirectly in a control system, therefore it is practical to exchange them for variables easily measurable and directly modifiable.

According to the velocity triangle in Fig. 2-10:

(2-16)

Using (2-11) the power coefficient can be given as a function of ϑ and λ instead of angles α and δ. From Fig.2- 13 it is apparent, that the advisable ϑ pitch angle values are 1ô~ 4ôduring normal operation in the interest of maximal energy capture, but during starting (or at low λ and w) higher torque and power can be extracted if ϑ≈15ô~30ô is set. To attain this the blades must be able to rotate around their lengthwise axis (variable pitch blade). (ϑ is the angle between the chord line and plan of rotation, thus it is measurable and adjustable directly, while the λ tip speed ratio can be set via the wT angular speed by the speed control of generator or by the change of the number of pole pairs.

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chapter 1.

The PT-v and MT-wT curves of present day HAWTs can be seen in Fig. 2-14. At the vi cut in wind velocity the output power of the turbine exceeds a little bit the no load mechanical and electrical losses of the system

“behind” the turbine. vn the rated wind speed is the lowest velocity at which the turbine can deliver its PTN rated power. The highest wind speed during operation, determined by safety, is designated with vmax.

In the first (I) region (vi≤v≤vn), the aim of control is to extract the maximum energy/power from the wind. This can be realized with a control: (CP)max →λopt →wopt , which means an angular speed regulation, proportional with the wind speed according to (2-8).

In the second (II) region (vn<v≤vmax) , in the case of grid connected operation a power control (PT=PTn=constant) is used. The purpose of this control is to protect the generator, power electronics, transformer etc. The vn rated speed is determined by economic standpoints, knowing the wind conditions of the given site.

The increase of this wind speed can be justified up to the value where the extracted additional energy covers the cost of additional investment increasing with vn. (The usual value of vn are: 10~13m/s.) It has to be considered too, that the occurrence of high wind speed is lower with increasing wind velocity.

In island operation (in the II region) the system has to control the power of the turbine respective of the load.

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chapter 1.

In the third (III) region the wT≤wmax speed limitation control law is in use for safety reasons, in order to prevent the damage caused by the large centrifugal forces. This control law can be interpreted by Fig. 2-14b. In this figure the MT-wT characteristics of a turbine without control are represented for different wind speeds. This graph shows, that a given turbine has increasing starting torque with growing wind velocity. This cannot be seen from the values of CP and output power in Fig. 2-13, because in the case of wT→0 the values of MTwT=PT→0. It is apparent, that the points of (CP)max does not coincide with points of MTmax.

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chapter 1.

We can understand the necessity and possibilities of power control in the II region by the assistance of Fig. 2-15.

Let us ignore the mechanical and electrical losses in the equipment connected after the turbine.

In the first (I) region the power contained in the wind surpasses the extractable power exactly by the aerodynamic losses. Above the vn rated wind speed the turbine would deliver power higher than the rated one, if the control theory were the same as in the first (I) region. This surplus power must not be let into the connected system to avoid overloading. That is the turbine has to be controlled to PTn=constant. This aim can be achieved by the following methods:

●yawing the nacelle/tilting the wind turbine, ●changing the pitch angle (ϑ or β),

●by stall control (with appropriate design of airfoil).

In the high power range generally the last two methods are used, while in the smallest units the region of constant power control may be absent because of economic reasons.

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chapter 1.

Hitherto, e.g. during the deduction of (2-3) and (2-6) it has been supposed that the wind direction is perpendicular to the rotation plane (A) of the turbine. If the turbine is yawed (view from above), or it is tilted (side view) according to the sketch in the right upper corner of Fig. 2-16, then the effective cross section of flow of the rotor is reduced in accordance with Acosγ. Let us introduce the notification: CP *=CPcosγ, so the CP *

values in Fig 2-16 can be obtained for different γ values according to measurements. It must be noted that the reduction of CP* is larger than that determined by cosγ for high γ values (because of consequent blade stall).

In the following we shall only deal with yawing.

What can yawing be used for?

a) It can be used to realign the rotor axis in response to change in wind direction (to follow it). It is peculiar to this control, that an angular error of 5^oresults in a power drop smaller than 1%, that is the realignment does not have to be too rapid or too precise. For this purpose rotor tilting is not suitable.

b) Is it appropriate for power control in the II. range?

The wind velocity changes more frequently and more quickly than the direction of wind. In addition to this in certain cases (e.g. grid failure) even more rapid intervention would be required. So it can be established, that this control method is not suitable for this purpose in the high power range. Viz. in the case of a 850kW wind power plant the mass of nacelle is 22t, and the mass of the turbine is 10t. In the latter case the equivalent radius of the centre of masses can be estimated about 15m. It can be seen that a huge momentum of inertia would be rotated, which requires a very large amount of power and exerts huge strain (with accompanying enormous costs), or one could be satisfied with slow rotation which results in wind energy loss and considerable additional stress of the blades in the case of a large error of angle. Besides these the turbine has to take a considerable angle of rotation to achieve a significant change in power. Viz. 50% of power change requires γ>40^o change in the angular position. The yawing of the rotor is rarely used for the control of power and only in the low power range.

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chapter 1.

The technical solution required to realign the rotor axis to wind direction can be studied in Fig. 2-17. At the top of the tower a roller yaw bearing or a gliding yaw bearing makes the turning of the nacelle possible (this bearing can not be seen in Fig. 2-17). A yaw ring (or gear rim) is fastened to the top of the tower, too (its teeth may be located at the inner or the outer cylindrical face). One or several motors fastened to the machine bed in the nacelle are coupled via planetary gear-box (yaw gear) and pinion to the yaw ring fixed to the top of the tower.

The resultant gear ratio is selected to make the nacelle rotate with 0.5^o/s angular speed during the operation of motors.

The orientation of the nacelle to the wind direction can be solved by position control, in which a wind vane set up on the top of the nacelle provides the reference value, while the feedback signal is given by a position sensor of the nacelle. The system can be spared from the unnecessary stresses of frequent small rotations if the control system does not have to intervene under a given error angle (under a threshold).

The other problem that has to be solved is the transfer of electrical energy from the generator (rotating together with the nacelle) to the transformer (and other equipment) fixed to the tower or the ground. The most general solution of this problem provides a flexible connection with long, slack, twistable cables. Today’s 50~120m- high wind power plants require long cables anyway, but the number of revolutions in one direction has to be limited up to 1~3 (this value has to be sensed separately).

In the high power range (in the case of high cable currents) other solutions may be required because of the more rigid cables. Application of slip-rings and brushes are possible, and in these cases there is no need to limit the

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chapter 1.

The theory and space conditions of blade pitch control are outlined in Fig. 2-18. This power control may be considerably quicker than the one implemented by yawing. When controlling the blade pitch one has to consider the moment of inertia of each blade concerning the lengthways axis. This moment of inertia is much less than the one concerning the vertical axis of yawing (e.g. tB<<R) and it is unnecessary to rotate the nacelle of large mass. The swifter response is also promoted by the fact, that e.g. only a 7^o pith angle change is required to reduce the output power by 50% (around λ≈7). The equation of motion for one blade:

(2-19)

Here the quantities are:

Jl: moment of inertia of one blade concerning to the lengthways axis, β: blade pitch angle (between the chord line and rotor axis),

k: damping coefficient, consisting of air and bearing friction, Md: driving (motor) torque for one blade,

Mtorsion: torsion moment (with restraining effect).

The torsion moment can have different components. The origin of these are that neither the centres of masses of blade elements nor the points of attack of dFE elementary lifting forces are located on the longitudinal rotation axis of the blade. Further reason may be the flexible change of form of blades.

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chapter 1.

Pitch regulation systems may be of different kinds.

1. Individual drive: each blade has an own driving motor. This mechanism can cause aerodynamic asymmetry between the blades.

2. Joint drive: with one motor. This solution requires a more complicated mechanism.

3. Hydraulic regulation mechanism: are used at low and medium power levels. This positioning system is able to exert the holding torque without extra energy expenditure. Blade return is by springs acting in the opposite direction, which provides a safe operation if any emergency arises.

4. Electrically driven pitch regulation can furnish the rotation in both directions. This drive can usually exert the necessary holding torque only under continually taking power. (Both, the hydraulic and electrical systems require an external energy source.)

The rated power of the drive used for positioning (considering emergencies, too) is usually 0.05Pn, which may be 50kW in the case of a turbine of 1MW. One could see that a 0.5Pn change can be achieved even with a 7^o change in angular position.

The pitch angle control demands quick and accurate intervention, so it requires a position control at servo level.

The block diagram of such a high-standard servo drive can be seen in Fig. 2-19.

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chapter 1.

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The pitch angle control is primarily used in the II. region, when the βa reference value is prescribed by the error signal of an outer power control loop.

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chapter 1.

One could see in Fig. 2-11, that the CE/CV lift-drag coefficient ratio (and Ft, MT and PT) is advantageous (high) as long as the α angle of attack is small. This angle can be changed indirectly via the ϑ pitch angle, too if the blade is able to turn as it was supposed in the previous 2-5-2 paragraph . But let us now examine wind turbine with fixed rotor blade. The wind conditions are sketched in Fig. 2-21.

Let us suppose there is an induction generator connected directly to the grid of constant frequency, and the wG≈constant generator angular speed forces wT≈constant angular speed on the turbines via gear-box. The fixed (not turnable) rotor blade means a ϑ=constant pitch angle, the value of which has to be chosen according to the aim introduced in the following. Based on the relation in (2-20) the δ angle of the wind speed triangle has to increase when v1 wind velocity grows, and the α angle of attack also has to increase because of ϑ=constant. In the case of proper design it results in the limiting of turbine power.

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chapter 1.

The physical explanation of the stall control can be studied in Fig. 2-22. At low wind speed the larger part of the lifting force (and the torque) is created by the suction on the back side of the blade. This is illustrated with the concentrated stream lines which are very close to the surface of the blade. In this speed range the designers strive for the best attainable aerodynamic efficiency.

At higher wind speed (above vn) at a given value of angle of attack some part of the flow unsticks from the back side of the blade, starting from the tail of the blade and it creates turbulence. This results in the decrease of the CE coefficientof the lifting force and the increase of the CV coefficient of the drag force. With increasing wind velocity the separation point and so the zone of vortex progresses towards the nose of the blade progressively, so the CE/CV ratio keeps dropping. The designers select the airfoil and the fix ϑ angle so as to decrease the aerodynamic efficiency considerably when above the rated wind speed and this way the PT turbine power remains nearly constant despite the increase of wind velocity.

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chapter 1.

According to Fig. 2-23 it is not possible to achieve such precise power control with stall control as with pitch angle control. Because of this the wind power plant has to be designed for a power exceeding the rated power with a few percents.

This control method is simple, cheap and reliable. The limitation arises automatically with the increase of the wind speed and it does not require any mechanism or triggering. It is used up to D=50m diameter (up to a maximum of 1.5MW) because above this size considerable vibration arises on the turbine blade which can cause fatigue in the turbine during a short time. Because of the same reason the synchronous generators with stiff speed-torque characteristic cannot be applied for the purpose of stall regulation.

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chapter 1.

The types of wind power plants used present day can be seen in Fig. 2-24. Within these types several slightly different configurations exist, which will be mentioned only in a few cases.

Common characteristic of the four types is the need for a gear-box with two or three-stages between the turbine and generator. They are also alike in the sense that most of the generators are made with a line voltage of 690V (effective) because of practical reasons. Therefore a transformer has to be inserted to raise the generator voltage to the level of medium voltage distribution network, or to the level of the voltage of power-collection system of wind farms of 20~25kV.

2-6-1. Type “A” wind power plants

In this type a squirrel-cage induction generator is directly (without frequency converter) connected to the grid.

For the sake of low rotor winding loss machines with low slips (with low rotor resistances) are used. Because of the constant line frequency the rpm of the generator can be regarded about constant in spite of the 1~2% slip.

Since the induction generator consume reactive power, it is advisable to compensate this with capacitor banks within the wind power plant.

In type “A” wind power plants generally fix rotor blades and stall-control are used. This type is usually called

“Danish type” wind power plant. Among others Nordex, Bonus, Ecotechnia and Siemens manufacture this type of wind power plants.

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chapter 1.

In Fig.2-25. a general block diagram of wind turbine systems can be seen, from which let us leave out the frequency converter (designated with F.V.) for the time being.

1. Because of the fH→wG→wT constraint, according to (2-8) the λopt and (CP)max can only be attained at one v wind speed.

2. Because of the rigid system the torque and power impulses can progress in both directions practically without any damping, which can cause great mechanical and electrical stress (and the system requires excessive oversizing).

3. To avoid this harmful property, despite the doubly-side constraint, the rigid system has to be “split”

somewhere, partly or entirely. Fig.2-25. illustrates 1), 2) and 3) possibilities. The manufactures seldom use the possibilities of discrete change of gear ratio. This is because the high power gear-boxes are sensitive and vulnerable parts of the wind power plants even in normal cases. Earlier constructions offered the selection between two gear ratios.

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chapter 1.

In the case of type “A” wind turbines the “split” of the system is realised in the generator (at the place indicated with 2). The following implementations are possible.

1. The stator windings of the generator are manufactured with centre tapping, which makes possible a 1:2 speed ratio change in the (synchronous) angular speed by the Dahlander theory. Unfortunately this ratio is not optimal from the point of view of energy extraction.

2. Two generators with a different number of poles can be connected to the gear-box alternatively (one is a full- load generator the other is a “weak” wind generator). The mechanical part of this system is complicated and expensive.

3. There are two windings in the generator stator with different number of poles (e.g. 2p=4 and 2p=6). Then the exploitation of the stator is poor, but the power of the winding used at lower rpm is considerably lower than the rated one. The b) and c) systems are identical in respect of (electrical) circuit diagram, so Fig. 2-26 can concern both cases.

According to Fig. 2-26 the change of number of poles only improve the energy extraction partly, as the optimal operation can only be achieved at two discreet wind speeds. At other wind velocities the wind turbine operates with CP<(CP)max. At lower wind speed the higher (2p=6), at higher wind speed the lower (2p=4) number of poles are more favourable from the point of view of energy extraction.

The rigidity of the system has remained the same at one given pole number, thus this solution does not reduce the large mechanical and electrical stress. Phase angle control of thyristor-pairs is used for soft start and during the change of number of poles.

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chapter 1.

In type “B” wind power plants slip ring induction generators are used. The equivalent value of the outer resistance introduced into the rotor circuit can be controlled by power electronics according to the requirements.

It is well known, that the speed-torque characteristic of the generator can be made “soft” by increasing the rotor resistance. That is larger Δn rpm change (or Δw change) belongs to the same ΔM torque change. The control of the rotor resistance has to be realized in a way that it permits the increase of rpm when the wind speed is increasing, and thus the storage of extra energy in the form of increased kinetic energy of the rotating masses.

During decreasing wind speed the wind power plant delivers this extra energy to the grid, so it “filters” or

“smoothes” the electric power of the generator. (This provides a better quality of energy.) Further it reduces the stress on the equipment. At stable wind speed the value of inserted outer rotor resistance is low for the sake of high efficiency.

Type “B” wind power plant thus reduces the stress but it spoils the efficiency a little bit because of the increased rotor resistance. The maximum relative value of outer rotor resistance may be 10%, which makes a 10% rpm change possible. The firm Vestas manufactures this type in brushless form (V27, V34,V47 types).

Soft starter and capacitor bank for compensation of reactive power requirement are necessary similarly to type

“A”.

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chapter 1.

Type “D” wind power plants can be built with permanently excited or separately excited salient pole synchronous machines or with squirrel-cage induction generators. The stators of the generators in all three cases are connected to the grid via frequency converters. The frequency converters have to be rated to the nominal power of the wind power plant, this makes possible a 100% change of rpm (from w=0 up to wmax). In the case of synchronous generator (in particular with permanently excited generator) the gear-box can be left out (gearless solution), because the synchronous generator can be built with a high number of poles. These generators have ring shaped rotors and stators (see Fig. 2-28a). Intermediate solution is also possible, when instead of the usual 2~3 stages there is a gear-box with only 1 stage and the generator has medium speed (and number of poles).

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chapter 1.

The application of frequency converter eliminates the harmful properties mentioned in paragraph 2-6-1, and it possess the following advantages compared to type “A” wind power plant.

1. It guaranties the λopt and (CP)max in all points of region I., and therefore it makes possible the extraction of more energy (see Fig. 2-28b).

2. vi can be reduced by lowering the aerodynamic losses and the no-load losses of the generator.

3. It provides for a flexible connection between the turbine and grid, because the rpm/frequency is adopted to the input power/torque, and it conserves their peak values mainly as the kinetic energy of the turbine. This means lower stress and energy with higher quality.

4. The starting procedure/connection to the grid is easier.

5. At low wind speed (in region I.) the noise level is lower.

6. The active and reactive power can be quickly and exactly controlled (see later).

The use of the frequency converter has some disadvantageous consequences, too.

1. It is a more expensive system.

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chapter 1.

Only the right hand side part of the block-scheme has been changed compared to Fig. 2-20. The line-side converter of the frequency converter is synchronized from the grid, while its machine-side converter gets synchronizing signals from the generator. The synchronizing signal is the line voltage in the first case while it may be the flux of the generator in the latter case. So the generator and the machine-side converter mutually determine their fG frequency for each-other, and this self-controlled system can not be dropped out of the synchronism. The wG angular speed can be changed via the λopt controller and its subordinated torque/current controller to achieve optimal energy extraction in region I. The simplest solution of the outer angular speed control loop can be seen in Fig.2-30.

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chapter 1.

Theoretically the PT=PTn=constant control in the II. region can be implemented by means of frequency converter by imposing the wT>wTopt and λ>λopt values on the system (see A6 ,A7 and A8 points in the Fig .2-31.).

This control method requires enormous over-scaling of the rotating units, considering, that the stress caused by the centrifugal force increases with the square of the angular speed. In contrast with this, by pitch angle control the power of B6 ,B7 and B8 operating points at v6 ,v7 and v8 wind speeds can be controlled down to PTn rated value (in the point N). That is, the wT angular velocity remains constant (e.g. the rated value). According to Fig. 2-29 generally both the frequency converter and pitch angle control are required. The further advantages of the joint applications are also supported by the following facts:

1. in the case of emergency they constitute the reserve of one another, 2. easier and better starting up can be achieved,

3. the frequency converter can increase the lifetime of the pitch controller significantly.

The latter advantage can be realized in the II. region, if the cooperation of the two controllers are based on the following.

1. The frequency controller handles the small and quick changes. The intervention of the frequency controller is rapid and it consists of components which do not wear out. The rotating parts have to be rated for an angular velocity that is higher than the nominal value only with Δw=10%.

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chapter 1.

Type “C” wind power plants are constructed with doubly fed asynchronous generator. The stator windings are connected to the grid directly while the rotor windings are joined up to it via a frequency converter.

Considering the flow directions of the motor operation the positive sign, the Pm ,Ps and PH powers and the M torque have a negative sign because of the (real) flow direction of generator operation. In motor operation region the slip determines how the air-gap power (about equivalent with the PS stator power) is divided into mechanical power and rotor power loss:

(2-22a,b)

In generator operation mode (in wind power plants) the Pm mechanical (or shaft) power has to be regarded as a given quantity which is determined by the wind speed at any time. Rearranging (2-23):

(2-23a,b)

In generator mode of operation of wind power plants one can suppose, that │s│<1, namely the denominators of (2-23) are positive. It follows from this, that because of the Pm mechanical power taken from the shaft the sign of Pl is also negative, while the sign of PR is positive above the synchronous speed, and is negative under the synchronous speed depending on the sign of the slip.

The largeness and the sign of the PR power can be prescribed by the control of machine–side converter and therefore one can prescribe the slip and the rpm (e.g. under or above the synchronous speed) of the doubly-fed induction generator according to (2-23b).

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chapter 1.

The high power rating frequency converter is one of the most expensive part of the wind power plants so it is important to design it with lower power. If the own (inner) power loss of the rotor is neglected, then PR means the power flowing through the frequency converter according to which the design has to be planned. The rpm should change proportional to the wind speed in the vi≤v<vn I. region. Considering the usual wind conditions and the economic indicators in general:

(2-24)

that is an rpm region with about 1:2 ratio is required. If the slip were changed in the ±0.3 range as first approximation (wmin≈0.7, wmax≈1.3), then the frequency converter (in the rotor circuit) should be design for 30%

of rated power. This is a huge economic benefit in comparison with type “D” wind power plants where the frequency converter has to be designed for the full (100%) rated power, as it was already mentioned.

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chapter 1.

The previous sequence of ideas would only be valid in the case of MG=const., Pl=const. If the losses of the drive train of the wind turbine is disregarded in the I. region, according to (2-6):

(2-25)

By the former discussion we attribute a negative sign for Pm, and we handle the k constant as a positive quantity, so:

(2-26) Using

(2-27)

the active power passing through the frequency converter:

(2-28)

If we select the synchronous angular speed and the kW13 mechanical power belonging to it as base values, then the required power in per unit system is:

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chapter 1.

(2-29)

The P’R-w’G relation seen in Fig. 2-33 suggest that we do not locate the I. region symmetrically around the synchronous speed but we should place it similar to Fig. 2-33.

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The numerical power relation of a doubly–fed induction generator are represented in Fig. 2-34 and Fig. 2-35 according to (2-23a,b) formula. For the sake of simplicity it is assumed in both cases that the total loss of the machine is about 10% of the PS stator power. In both cases the Pm≈PT power was considered as the base value (that is 100%), the difference of their absolute values are made perceptible by the difference of Pm sizes in the figures according to (2-25).

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chapter 1.

Under synchronous speed the system operates with a lower efficiency (e.g. with 87%), whereas above the synchronous rpm it operates with a higher efficiency (e.g. with 93%). The explanation for this is that in the case of s=0.3 the PR power circulates only within the system causing loss but this power is not delivered to the grid.

On the other hand the power delivered to the grid is PH=PS+PR in the case of s= -0.3. This difference between efficiencies limits the portion of the I. region that extends into the domain under W1 synchronous speed (see Fig.

2-33).

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chapter 1.

The frequency converters used in wind power plants generally contain two-level voltage source inverters, and the semiconductor applied in it is the IGBT. In new models, in the highest power range three-level voltage source inverters are already used as well as IGCT as semiconductors.

Hereafter the machine-side converter will be marked with GÁ and the grid-side converter with HÁ. Today the most frequently used control theory is the field-oriented control in both GÁ and HÁ converters. It makes it possible that the two perpendicular components of the three-phase current vector can be controlled independently from each other via the generator voltages (and with proper compensation). The imaginary perpendicular (Descartes) reference frame can be fixed either to the synchronously rotating voltage vector or to the flux vector. With one current component (IW) the active power (P) with the other component (Im) the reactive power (Q) can be controlled.

In type “D” wind power plant (Fig. 2-36) by the control of IW current component of the GÁ converter through the PHa→M→wG→wT relations fixed λopt and (CP)max values can be achieved at any wind speed of the I. region.

The QG reactive power or the US stator voltage of the generator can be controlled by the Im current component.

The former results in the cheapest generator in the case of cosφG=1, while the latter makes the design for constant voltage stress possible. These control methods do not affect directly the reactive power exchange with the grid.

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chapter 1.

The QH grid reactive power consumption or supply can be controlled by the Im current component of HÁ converter. The Ue voltage in the intermediate circuit is kept at constant value with the IW current component. One reason for this is that the controllability of HÁ converter is ceases if the Ue dc voltage is not higher than the pick line-to-line voltage of the grid. The other reason is the following: if we neglect the power losses the balance between the power produced by the turbine and the power delivered by the generator to the grid can most simply and most sensitively be guaranteed by the constant value of Ue voltage,. Namely the current of the GÁ converter charges and the current of the HÁ converter discharges the charge of Ce condenser (see Fig. 2-37).

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chapter 1.

The control tasks of HÁ converter of a type “C” wind power plant are nearly equivalent to the tasks of HÁ converter of a type “D” wind power plant. The difference is that this converter controls only the QR reactive power coming from the rotor side of the machine, which is generally only one part of the total reactive power delivered to the grid.

In this type the active current component of GÁ converter is also used to keep (CP)max via the slip or speed control. Either the Im reactive current component is applied to precisely cover the required reactive power of the induction generator or with a higher QHa reference signal this can provide a way to deliver reactive power to the grid, too.

45. 45. dia

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chapter 1.

The reasons for the necessity of hybrid system are as the following.

1. The problems of power system control (there is a need for energy storage).

2. The generating capacity of Hungary is inflexible (e.g. 3000MW is base load power plant).

3. In the case of calm or low wind speed the power plants operating expensively also have to be used.

4. In the case of high wind speed the power plants operating economically must be cycled down because of the feed-in-tariff. It has safety risks and it can result in shorter life span.

5. The output power of the wind turbines can only be controlled downwards (under the actual maximum extractable power).

6. The wind power plants have a higher │dP/dt│ gradient than the thermal power stations.

There is a growing demand for hydrogen:

1. in the hydrogen-fuelled vehicles (in environment-friendly transport/traffic), 2. in other cases (chemical industry, locally generated electrical/heat energy).

Today’s the most frequently used electrolyser types are:

1. alkaline electrolyser (with KOH liquid electrolyte), 2. solid proton exchange membrane (PEM) electrolyser, 3. electrolyser with solid oxide membrane (SOEC).

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chapter 1.

Reaction on the anode is:

Reaction on the cathode is:

The actual voltage range of an electrolyser cell is 1.4~2V during operation, which is a nonlinear function of the current density, the temperature and the pressure (see Fig.2-40b).

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chapter 1.

The low voltage of electrolyser cells has to be adjusted to the usual voltage levels of power supply. The rough adjustment can be achieved by the series connection of the cells but it has special drawbacks and limits, too. The fine adjusting and current control of the electrolyser is possible with buck dc/dc converter (see Fig. 2-41).

The energy can be stored in the hydrogen (in chemical form). The energy stored in the hydrogen (HHV, the higher heating value) is 142MJ/kg, which far exceeds the energy content of other energy carriers (fuels). E.g. the similar characteristic of natural gas and gasoline is 57MJ/kg and 45MJ/kg respectively.

During the peak load hours the H2 can be converted into electrical energy (in the wind power plant) with the aid of fuel cells, or e.g. it can be transported to the filling stations for cars and buses.

48. 48. dia

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chapter 1.

The most frequently used hydrogen fuel cells are the following (with their English acronyms):

a) proton exchange membrane fuel cells: PEMFC, b) alkaline electrolyte fuel cells: AFC,

c) direct methanol fuel cells: DMFC, d) phosphoric acid fuel cells: PAFC, e) molten carbonate fuel cells: MCFC, f) solid oxide fuel cells: SOFC.

The application fields, advantageous properties and usual power ranges of applications of the above listed fuel cells can be seen in Fig. 2-42.

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chapter 1.

The theoretical construction of a PEM fuel cell is represented in Fig. 2-43.

The reaction at the anode is:

2H2→4H⁺+4e^-, at the cathode is:

O2+4H⁺+4e^-→2H2O, the whole reaction is:

2H2+O2→2H2O.

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chapter 1.

The ideal no-load (open circuit) voltage of PEM fuel cell is 1.23V at about 20~25^oC (in consistent with Fig. 2- 40) if the product is water in liquid form (HHV). In the case of fuel cells all the irreversible reactions (over- voltages), which required higher input voltages than 1.23V in the normal operation of the electrolyser at a given current density, will arise with the opposite sign and will reduce the output voltage.

The voltage-current density characteristic of a PEM fuel cell can be seen in Fig. 2-44.

The nominal (working) voltage is: 0.6V≤U≤0.8V, typically U≈0.65V.

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chapter 1.

This low voltage (voltage) source has to be adjusted to the usual voltage level of Ue by the series connection of cells and step-up dc/dc converter (see Fig. 2-45).

52. 52. dia

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chapter 1.

Fuel cells are sensitive components, so they need to be protected against e.g. : 1. rapid change of load,

2. high current density,

3. low frequency current ripple (fν<400Hz) with considerable amplitude (iν<4%).

To provide this protection it is necessary to have controllable power electronics (see Fig. 2-45) and energy storage with high enough dynamics. Generally accumulators or super capacitors fill the latter role.

Because of the requirement of a two directional energy flow a two quadrant dc/dc converter has to be used – e.g.

visible in Fig. 2-46.

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chapter 1.

The hydrogen technology is easiest to fit to type “D” wind power plant, namely to the intermediate dc circuit (see Fig. 2-47). In this case the rated powers and control methods of the converters of the frequency converter demand insignificant modifications.

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chapter 1.

Keeping the output power at a constant value (within a short period, e.g. within 15 minutes) may be one possible control strategy of a wind turbine-hydrogen hybrid system outlined in Fig. 2-47. Then, according to Fig. 2-48 during the high wind speed periods the surplus power is utilized in the electrolyser while during the low wind speed periods the missing electrical power is replaced by the fuel cell stack. In the interest of efficient operation the electrolyser and the fuel cell stack are controlled alternately (with output overlap, or a changeover hysteresis in the control loop).

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chapter 1.

In the case of type “A” and “B” wind power plants the hydrogen sub-system can only be joined to the alternating current grid (distribution network, see Fig. 2-49). This arrangement can be used at any type of wind power plant, which makes it profitable with the help of possible mass production.

This model promotes a hydrogen sub-system separated from the wind power plant.

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chapter 1.

57. 57. dia

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chapter 1.

58. 58. dia

Hydroelectric plants provide about 20% of the world’s annual electric output. Hydro power plants can be classified in many ways (see Table 3-1).

The three largest hydroelectric plants of the world are (2010):

●Three Gorges (China): 32*780MW, ●Itaipu (Brazil-Paraguay): 20*720MW, ●Guri 2 (Venezuela): 10*770MW.

The greatest hydroelectric plant of Europe is:

“Iron-gate” (Romania-Serbia): 10*190MW.

The maximum efficiency of turbine (today): ηT=96.5%.

The maximum efficiency of generator: η G=98.5%.

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chapter 1.

According to the head of water the different types of hydroelectric plants can be seen in Fig. 3-1, these are:

1. low head: with barrage, run of river type, with Kaplan or propeller turbine, 2. medium head: river or lake with dam, with Francis turbine,

3. high head: with high reservoir on a mountain, with Pelton turbine,

4. low head and low volume flow rate (small-scale hydroelectricity): run of river type with bulb installation (it can be seen in Fig. 3-5).

60. 60. dia

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chapter 1.

Types of turbine runners.

a./ Francis, b./ propeller, c./ Pelton, d./ Kaplan.

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chapter 1.

In Fig. 3-3 the structure of a Francis turbine can be seen in “cut-away” form (from the outside towards the inside):

● volute, also called a scroll case (with decreasing cross-section), ●guide vanes (are pivoted to adjust the flow rate),

●runner blades (with fix blade angle).

By turning the guide vanes the flow rate can be adjusted but at the same time the efficiency of the turbine will be changed, too. The turbine position is generally vertical. The blades deflect the water flow (action force), and the water exerts a force on the blades (reaction force), which maintains the rotation of the runner. Thus it is a so called “reaction” turbine.

The pressure drop of the water passing through the turbine accounts for a significant part of the total delivered energy.

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chapter 1.

If the potential possesses large volume flow with low head, then the axial flow turbine (propeller or Kaplan) with a large entering area is used. To divert the water flow appropriately the blade has to have a gradually twisted shape, similar to the propeller. The propeller turbine has fixed blades on the runner so it is simple and inexpensive. The possibility of rotation of runner blades is necessary to achieve high efficiency if the required power may change significantly (this is the Kaplan turbine). These turbines fall within the group of “reaction”

turbines. The decisive difference compared to the Francis turbine is that the outer parts of the runner blades move faster than the water (at least twice as fast).

63. 63. dia

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chapter 1.

The tubular turbines can be applied advantageously in the case of low head and low flow rates (small-scale hydroelectricity). This name is a collective noun, which includes turbines possessing the following characteristics:

● their shafts are horizontal or gently inclined,

● there is no scroll case (volute), the duct is horizontal, ● the used runner is propeller or Kaplan,

●their generators have a special construction (e.g. rim generators).

One particular type of tubular turbine is the bulb installation, in which the generator is sealed in a bulb-shaped enclosure mounted in the flow. The water has to flow around the large bulb (see Fig. 3-5.). This type of turbine is used in the “Kisköre” hydroelectric power plant.

These types of turbines are also used in tidal power generation.

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chapter 1.

The extractable power from a hydro power plant in [kW] is:

(3-1) Here:

g: gravitational acceleration in [m/s²], ρ: water density in [kg/dm³],

Q: volume flow rate in [m³/s],

ηT, ηG, ηTR: efficiencies (turbine, generator, transformer).

The nominal rpm of the turbines is determined by the hydraulic side:

(3-2)

where: nTn , PTn and H have to be substituted in [1/min], [kW] and [m] respectively.

NS the “specific speed” has been developed on the basis of economical and construction optimums, and it can be selected from tables (see Table 3-2.).

Table 3-2.

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chapter 1.

The generators of large-scale hydro power plants are multi-pole synchronous generators:

(3-3)

The rotor has a ring (or rim) form and has salient poles. The synchronous rpm are determined by the line frequency and the number of poles:

(3-4)

At high power level gear-boxes are not used, so:

(3-5)

As it was mentioned the rpm is determined by the hydraulic side, so the required number of poles is:

Electrical Systems of Renewable Energies

Electrical Systems of Renewable Energies

BMEVIVEM262

Károly Veszprémi Mátyás Hunyár

István Vajda

Electrical Systems of Renewable Energies

Table of Contents

Chapter 1. chapter 1.

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